Method for producing sintered ferrite magnet, and sintered ferrite magnet

ABSTRACT

A sintered ferrite magnet comprising (a) a ferrite phase having a hexagonal M-type magnetoplumbite structure comprising Ca, an element R which is at least one of rare earth elements and indispensably includes La, an element A which is Ba and/or Sr, Fe, and Co as indispensable elements, the composition of metal elements of Ca, R, A, Fe and Co being represented by the general formula of Ca 1-x-y R x A y Fe 2n-z Co z , wherein the atomic ratios (1-x-y), x, y and z of these elements and the molar ratio n meet the relations of 0.3≦(1-x-y) ≦0.65, 0.2≦x≦0.65, 0≦y≦0.2, 0.03≦z≦0.65, and 4≦n≦7, and (b) a grain boundary phase indispensably containing Si, the amount of Si being more than 1% by mass and 1.8% or less by mass (calculated as SiO 2 ) based on the entire sintered ferrite magnet, and its production method.

CROSS REFERENCE TO RELATED APPLICATION

This application is a National Stage of International Application No. PCT/JP2010/060280 filed on Jun. 17, 2010, claiming priority based on Japanese Patent Application No. 2009-156297, filed Jun. 30, 2009, the contents of all of which are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to a method for producing a sintered ferrite magnet, and a sintered ferrite magnet.

BACKGROUND OF THE INVENTION

Sintered ferrite magnets are used in various applications such as motors, power generators, speakers, etc. Known as typical sintered ferrite magnets are Sr ferrite (SrFe₁₂O₁₉) and Ba ferrite (BaFe₁₂O₁₉) each having a hexagonal M-type magnetoplumbite structure. These sintered ferrite magnets are produced from starting materials comprising iron oxide, carbonates of strontium (Sr) or barium (Ba), etc. by a powder metallurgy method at a relatively low cost.

Recently, to provide smaller, lighter-weight, higher-performance electronic parts for automobiles, parts for electric appliances, etc. for environmental advantages, sintered ferrite magnets are required to have higher performance. Particularly desired for motors for automobiles are sintered ferrite magnets having, in addition to high residual magnetic flux density B_(r), such high coercivity H_(cJ) as to avoid demagnetization by a demagnetizing field generated when thinned.

To provide sintered ferrite magnets with improved magnetic properties, proposals have been made to improve coercivity H_(cJ) and a residual magnetic flux density B_(r) by substituting part of Sr with a rare earth element such as La, etc., and substituting part of Fe with Co in the above Sr ferrite (for example, JP 10-149910 A, and JP 11-154604 A).

The Sr ferrite with part of Sr substituted by a rare earth element such as La, etc. and part of Fe substituted by Co, etc. (hereinafter referred to as “Sr—La—Co ferrite”), which is described in JP 10-149910 A and JP 11-154604 A, has excellent magnetic properties, finding various applications in place of conventional Sr or Ba ferrite. However, further improvement in magnetic properties is desired.

Also known for the sintered ferrite magnets together with the above Sr or Ba ferrite is Ca ferrite. It is known that the Ca ferrite has a stable structure represented by the formula of CaO—Fe₂O₃ or CaO-2Fe₂O₃, forming hexagonal ferrite by the addition of La. However, it has magnetic properties on substantially the same levels as those of conventional Ba ferrite, not sufficiently high.

To improve a residual magnetic flux density B_(r) and coercivity H_(cJ) as well as the temperature characteristics of coercivity H_(cJ), Japanese Patent 3181559 discloses Ca ferrite with part of Ca substituted by a rare earth element such as La, etc. and part of Fe substituted by Co, etc. (hereinafter referred to as “Ca—La—Co ferrite”), which has an anisotropic magnetic field H_(A) of 20 kOe or more, higher by 10% or more than that of Sr ferrite.

However, the Ca—La—Co ferrite described in Japanese Patent 3181559 has substantially the same B_(r) and H_(cJ) as those of Sr—La—Co ferrite, despite a higher anisotropic magnetic field H_(A) than that of the Sr—La—Co ferrite. In addition, having an extremely poor squareness ratio, it does not meet high coercivity and a high squareness ratio simultaneously, failing to be used for various applications such as motors, etc.

With respect to the Sr—La—Co ferrite proposed by JP 10-149910 A and JP 11-154604 A, JP 2007-123511 A proposes a method for improving H_(cJ) while maintaining B_(r). In general, a sintered ferrite magnet is produced by the steps of (1) mixing iron oxide with carbonate of Sr or Ba, etc. as starting materials, (2) calcining a starting material mixture for ferritization to obtain a calcined body, (3) pulverizing the calcined body to powder, (4) molding the powder to a green body, and (5) sintering the green body to obtain a sintered body. JP 2007-123511 A discloses a pulverization step comprising a first fine pulverization step, a powder heat-treating step and a second fine pulverization step, by which the percentage of crystals as large as 1.1 μm or less is made 95% or more, improving H_(cJ).

Though H_(cJ) is improved by the method of JP 2007-123511 A, cost increase is unavoidable due to increase in the number of production steps. Particularly, Sr—La—Co ferrite containing expensive elements such as Co, La, etc. as indispensable components suffers a higher material cost than conventional Sr ferrite. Higher production cost deprives sintered ferrite magnets of the most important economic advantages, failing to meet cost requirements in the market.

Various measures for improving performance have been conducted on the Ca—La—Co ferrite proposed by Japanese Patent 3181559. For example, JP 2006-104050 A proposes Ca—La—Co ferrite containing La and Co at predetermined percentages with the molar ratios of elements and the value of n optimized. WO 2007/060757 proposes Ca—La—Co ferrite with part of Ca substituted by La and Ba. WO 2007/077811 proposes Ca—La—Co ferrite with part of Ca substituted by La and Sr. However, the Ca—La—Co ferrites of JP 2006-104050 A, WO 2007/060757 and WO 2007/077811 fail to provide sintered ferrite magnets having high H_(cJ) satisfying demands in the market.

JP 2008-137879 A and WO 2008/105449 disclose a technology for improving H_(cJ) by using the pulverization step described in JP 2007-123511 A in the production of Ca—La—Co ferrite. Though the Ca—La—Co ferrites described in JP 2008-137879 A and WO 2008/105449 have improved H_(cJ), they doubly suffer increase in a material cost and a production cost as in JP 2007-123511 A, failing to meet cost requirements in the market.

Thus, sintered ferrite magnets meeting recent requirements of magnetic properties and costs have not been proposed yet.

It is known to add SiO₂, CaCO₃, etc. as sintering aids to sintered ferrite magnets, to change balance between B_(r) and H_(cJ), which are in a trade-off relation in the sintered ferrite magnets. To obtain high B_(r), it is effective to reduce the amounts of sintering aids acting as non-magnetic components in a range having liquid phase components necessary for sintering, or to increase the amount of CaCO₃ relative to that of SiO₂. However, such measures make it difficult to maintain fine sintered structures, resulting in low H_(cJ). On the other hand, to obtain high H_(cJ), it is effective to increase the amounts of sintering aids, or to increase the amount of SiO₂ relative to that of CaCO₃. However, such measures increase the amounts of non-magnetic components or lower sinterability, inevitably resulting in decrease in B_(r) and a squareness ratio H_(k)/H_(cJ), wherein H_(k) was the value of H at J of 0.95B_(r) on a curve of J (intensity of magnetization) to H (intensity of a magnetic field) in the second quadrant.

In conventional sintered ferrite magnets, particularly in Sr—La—Co ferrite and Ca—La—Co ferrite proposed recently, the amounts of sintering aids such as SiO₂, CaCO₃, etc. are generally as small as possible to maintain high B_(r). For example, JP 2006-104050 A describes that 0.3-1.5% by mass (calculated as CaO) of CaCO₃ and 0.2-1.0% by mass of SiO₂ are added preferably at the time of pulverizing a calcined body. WO 2007/060757 describes that 0.2-1.5% by mass (0.112-0.84% by mass when calculated as CaO) of CaCO₃, and 0.1-1.5% by mass of SiO₂ are added preferably at the time of pulverizing a calcined body. JP 2008-137879 A describes that 1.35% or less by mass of SiO₂ is preferably added to a sintering material.

However, JP 2006-104050 A, WO 2007/060757 and JP 2008-137879 A describe in Examples only Ca—La—Co ferrite magnets, in which the amounts of both SiO₂ and CaCO₃ (calculated as CaO) are 0.9% or less by mass. Because emphasis is placed on the improvement of the B_(r) of these ferrite magnets, the addition of SiO₂ and CaCO₃ in amounts of more than 0.9% by mass is not contemplated, providing no information about the magnetic properties (B_(r), H_(cJ) and H_(k)/H_(cJ)) of such Ca—La—Co ferrite magnets.

OBJECT OF THE INVENTION

Accordingly, an object of the present invention is to provide a sintered Ca—La—Co ferrite magnet having extremely improved H_(cJ) while maintaining high B_(r) and squareness ratio H_(k)/H_(cJ) without suffering increase in a material cost and a production cost, which can therefore be made thinner, and a method for producing such sintered ferrite magnet at a low cost.

DISCLOSURE OF THE INVENTION

As a result of intensive research in view of the above object, the inventors have paid attention to sintering aids added at a pulverization step, finding that in Ca—La—Co ferrite, (a) the addition of SiO₂ in an amount exceeding 1% by mass as a sintering aid specifically increases H_(cJ); (b) by adding 1% or more by mass of CaCO₃ depending on the amount of SiO₂ added, decrease in B_(r) and H_(k)/H_(cJ) can be avoided as much as possible; therefore, (c) a sintered ferrite magnet having high H_(cJ) with high B_(r) and H_(k)/H_(cJ) maintained, which has not been obtained so far, is produced. The present invention has been completed based on such finding.

The specific increase in H_(cJ) by the addition of SiO₂ in an amount exceeding 1% by mass is a phenomenon peculiar to the Ca—La—Co ferrite. In the Sr—La—Co ferrites described in JP 10-149910 A and JP 11-154604 A, the addition of SiO₂ and CaCO₃ in amounts exceeding 1% by mass provides only small increase in H_(cJ), accompanied by extreme decrease in B_(r) and H_(k)/H_(cJ) (see Example 3 below).

Measurement by the inventors using a singular point detection (SPD) method has revealed that Ca—La—Co ferrite in which the atomic ratio of Co is 0.3 has an anisotropic magnetic field H_(A) of 2.1 MA/m (about 26.4 kOe), higher than 1.8 MA/m (about 22.6 kOe) of general Sr—La—Co ferrite in which the atomic ratio of Co is 0.2. Namely, it is expected for Ca—La—Co ferrite to have higher H_(cJ) than that of Sr—La—Co ferrite. The details of the SPD method is described in Asti and S. Rinaldi, J. Appl. Phys., 45 (1974), pp. 3600-3610.

However, the Ca—La—Co ferrite magnets having a Co atomic ratio of 0.3, which are described in Examples of JP 2006-104050 A, WO 2007/060757 and JP 2008-137879 A, have H_(cJ) of about 400 kA/m (about 5 kOe) at highest. H_(cJ) of about 400 kA/m is obtained even in the Sr—La—Co ferrite having a Co atomic ratio of 0.2 (JP 2007-123511 A). Namely, though the Ca—La—Co ferrite magnets described in Examples of JP 2006-104050 A, WO 2007/060757 and JP 2008-137879 A have a higher anisotropic magnetic field H_(A) than that of the Sr—La—Co ferrite, they have substantially the same H_(cJ) as that of the Sr—La—Co ferrite, not improved to an expected level, and thus failing to exhibit potential inherent in the materials. The fact that H_(cJ) is specifically improved by the addition of SiO₂ in an amount exceeding 1% by mass in the present invention is a phenomenon peculiar to the Ca—La—Co ferrite, providing a remarkable effect of achieving properties close to potential inherent in the materials.

Thus, the method of the present invention for producing a sintered ferrite magnet comprising (a) a ferrite phase having a hexagonal M-type magnetoplumbite structure, and (b) a grain boundary phase indispensably containing Si, comprises

a step of preparing a calcined body of ferrite comprising a ferrite phase having a hexagonal M-type magnetoplumbite structure comprising Ca, an element R which is at least one of rare earth elements and indispensably includes La, an element A which is Ba and/or Sr, Fe, and Co as indispensable elements, the composition of metal elements of Ca, R, A, Fe and Co being represented by the general formula of Ca_(1-x-y)R_(x)A_(y)Fe_(2n-z)CO_(z), wherein the atomic ratios of Ca (1-x-y), the element R (x), the element A (y) and Co (z), and the molar ratio of n meet the following relations:

-   0.3≦(1-x-y)≦0.65, -   0.2≦x≦0.65, -   0≦y≦0.2, -   0.03≦z≦0.65, and -   4≦n≦7;

a step of pulverizing the calcined body to powder;

a step of molding the powder to a green body; and

a step of sintering the green body to obtain a sintered body;

before the pulverization step, more than 1% by mass and 1.8% or less by mass of SiO₂ being added to 100% by mass of the calcined body.

1-2% by mass (calculated as CaO) of CaCO₃ is preferably added to 100% by mass of the calcined body, before the pulverization step.

The amount of the SiO₂ added is preferably 1.1-1.6% by mass.

The amount of the CaO added is preferably 1.2-2% by mass.

The above (1-x-y), x, y and z and n are preferably

-   0.35≦(1-x-y)≦0.55, -   0.4≦x≦0.6, -   0≦y≦0.15, -   0.1≦z≦0.4, and -   4.5≦n≦6.

The above (1-x-y), x, y and z and n are more preferably

-   0.42≦(1-x-y)≦0.5, -   0.45≦x≦0.55, -   0≦y≦0.08, -   0.2≦z≦0.3, and -   4.8≦n≦5.2.

The above pulverization step preferably comprises a first fine pulverization step, a step of heat-treating powder obtained by the first fine pulverization step, and a second fine pulverization step of pulverizing the heat-treated powder again.

The sintered ferrite magnet of the present invention comprises

(a) a ferrite phase having a hexagonal M-type magnetoplumbite structure comprising Ca, an element R which is at least one of rare earth elements and indispensably includes La, an element A which is Ba and/or Sr, Fe, and Co as indispensable elements, the composition of metal elements of Ca, R, A, Fe and Co being represented by the general formula of Ca_(1-x-y)R_(x)A_(y)Fe_(2n-z)CO_(z), wherein the atomic ratios of Ca (1-x-y), the element R (x), the element A (y) and Co (z), and the molar ratio of n meet the following relations:

-   0.3≦(1-x-y)≦0.65, -   0.2≦x≦0.65, -   0≦y≦0.2, -   0.03≦z≦0.65, and -   4≦n≦7, and

(b) a grain boundary phase indispensably containing Si, the amount of Si being more than 1% by mass and 1.8% or less by mass (calculated as SiO₂), based on the entire sintered ferrite magnet.

The amount of Si is preferably 1.1-1.6% by mass (calculated as SiO₂), based on the entire sintered ferrite magnet.

The above (1-x-y), x, y and z and n are preferably

-   0.35≦(1-x-y)≦0.55, -   0.4≦x≦0.6, -   0≦y≦0.15, -   0.1≦z≦0.4, and -   4.5≦n≦6.

The above (1-x-y), x, y and z and n are more preferably

-   0.42≦(1-x-y)≦0.5, -   0.45≦x≦0.55, -   0≦y≦0.08, -   0.2≦z≦0.3, and -   4.8≦n≦5.2.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relation between the amount of SiO₂ added and a residual magnetic flux density B_(r) in the sintered ferrite magnet of Example 1.

FIG. 2 is a graph showing the relation between the amount of SiO₂ added and coercivity H_(cJ) in the sintered ferrite magnet of Example 1.

FIG. 3 is a graph showing the relation between the amount of SiO₂ added and a squareness ratio H_(k)/H_(cJ) in the sintered ferrite magnet of Example 1.

FIG. 4 is a graph showing the relation between the amount of SiO₂ added and coercivity H_(cJ) in the sintered ferrite magnet of Example 1.

FIG. 5 is a graph showing the relation between the amount of CaO added and coercivity H_(cJ) in the sintered ferrite magnet of Example 1.

FIG. 6 is a graph showing the relation between the amount of SiO₂ added and a residual magnetic flux density B_(r) in the sintered ferrite magnet of Example 2.

FIG. 7 is a graph showing the relation between the amount of SiO₂ added and coercivity H_(cJ) in the sintered ferrite magnet of Example 2.

FIG. 8 is a graph showing the relation between the amount of SiO₂ added and a squareness ratio H_(k)/H_(cJ) in the sintered ferrite magnet of Example 2.

FIG. 9 is a graph showing the relation between the amount of SiO₂ added and coercivity H_(cJ) in the sintered ferrite magnet of Example 2.

FIG. 10 is a graph showing the relation between the amount of SiO₂ added and a residual magnetic flux density B_(r) in the sintered ferrite magnets of Examples 1-3.

FIG. 11 is a graph showing the relation between the amount of SiO₂ added and coercivity H_(cJ) in the sintered ferrite magnets of Examples 1-3.

FIG. 12 is a graph showing the relation between the amount of SiO₂ and a squareness ratio H_(k)/H_(cJ) in the sintered ferrite magnets of Examples 1-3.

FIG. 13 is a graph showing the relation between CaO/SiO₂ and coercivity H_(cJ) in the sintered ferrite magnets of Examples 1-3.

FIG. 14 is a graph showing the relation between the amount of SiO₂ added and a residual magnetic flux density B_(r) in the sintered ferrite magnet of Example 4.

FIG. 15 is a graph showing the relation between the amount of SiO₂ added and coercivity H_(cJ) in the sintered ferrite magnet of Example 4.

FIG. 16 is a graph showing the relation between the amount of SiO₂ added and a squareness ratio H_(k)/H_(cJ) in the sintered ferrite magnet of Example 4.

FIG. 17 is a graph showing the relation between CaO/SiO₂ and coercivity H_(cJ) in the sintered ferrite magnet of Example 4.

FIG. 18 is a graph showing the relation between the amount of SiO₂ added and a residual magnetic flux density B_(r) in the sintered ferrite magnet of Example 5.

FIG. 19 is a graph showing the relation between the amount of SiO₂ added and coercivity H_(cJ) in the sintered ferrite magnet of Example 5.

FIG. 20 is a graph showing the relation between the amount of SiO₂ added and a squareness ratio H_(k)/H_(cJ) in the sintered ferrite magnet of Example 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[1] Sintered Ferrite Magnet

The sintered ferrite magnet of the present invention comprises

(a) a ferrite phase having a hexagonal M-type magnetoplumbite structure comprising Ca, an element R which is at least one of rare earth elements and indispensably includes La, an element A which is Ba and/or Sr, Fe, and Co as indispensable elements, the composition of metal elements of Ca, R, A, Fe and Co being represented by the general formula of Ca_(1-x-y)R_(x)A_(y)Fe_(2n-z)CO_(z), wherein the atomic ratios of Ca (1-x-y), the element R (x), the element A (y) and Co (z), and the molar ratio of n meet the following relations:

-   0.3≦(1-x-y)≦0.65, -   0.2≦x≦0.65, -   0≦y≦0.2, -   0.03≦z≦0.65, and -   4≦n≦7, and

(b) a grain boundary phase indispensably containing Si, the amount of the above Si being more than 1% by mass and 1.8% or less by mass (calculated as SiO₂), based on the entire sintered ferrite magnet.

The Ca content (1-x-y) is 0.3≦(1-x-y)≦0.65. When Ca is less than 0.3, the amounts of the elements R and A are relatively large, undesirably resulting in low B_(r) and H_(k)/H_(cJ). When Ca is more than 0.65, the amounts of the elements R and A are relatively small, undesirably resulting in low B_(r) and H_(k)/H_(cJ). The range of (1-x-y) is preferably 0.35≦(1-x-y)≦0.55, more preferably 0.42≦(1-x-y)≦0.5.

The element R is at least one of rare earth elements, indispensably including La. To obtain high magnetic properties, the percentage of La in the element R is preferably 50 atomic % or more, more preferably 70 atomic % or more, most preferably La alone except for inevitable impurities. because La is most dissolvable in the M phase among the elements R, the larger percentage of La provides larger effects of improving magnetic properties. The amount (x) of the element R is 0.2≦x≦0.65. When x is less than 0.2 or more than 0.65, low B_(r) and H_(k)/H_(cJ) are provided. The range of x is preferably 0.4≦x≦0.6, more preferably 0.45≦x≦0.55.

The element A is Ba and/or Sr. The amount (y) of the element A is 0≦y≦0.2. Although the effects of the present invention would not be deteriorated even if the element A were not added, the addition of the element A makes crystals finer in the calcined body with smaller aspect ratios, resulting in improved H_(cJ). The range of y is 0≦y≦0.15, more preferably 0≦y≦0.08.

The amount (z) of Co is 0.03≦z≦0.65. When z is less than 0.03, the addition of Co does not provide effects of improving magnetic properties. Also, because unreacted α-Fe₂O₃ remains in the calcined body, a slurry tends to leak from a mold cavity during wet molding. When z is more than 0.65, undesired phases containing a large amount of Co are formed, resulting in largely decreased magnetic properties. The range of z is preferably 0.1≦z≦0.4, more preferably 0.2≦z≦0.3.

Part of Co may be substituted by at least one of Zn, Ni and Mn. Particularly, the partial substitution of Co with Ni and Mn reduces production cost without lowering the magnetic properties. Also, the partial substitution of Co with Zn improves B_(r), despite slightly reduced H_(cJ). The total amount of substituting Zn, Ni and Mn is preferably 50 mol % or less of Co.

In the Ca—La—Co ferrite, increase in the amounts of Co and La essentially results in improvement in H_(cJ). However, because Co and La are rare metals, which are rare and expensive, their contents are desirably as small as possible for the purposes of reducing resource consumption and lowering the prices of sintered ferrite magnets. Because H_(cJ) can be extremely improved while maintaining high B_(r) and squareness ratio H_(k)/H_(cJ) by the present invention, the amounts of Co and La can be reduced to provide magnets having the same H_(cJ) as that of the conventional sintered Ca—La—Co ferrite magnets. It makes it possible to obtain H_(cJ) on a practical level even in compositions containing Co at an atomic ratio of 0.2, whose practical use has been difficult because high H_(cJ) cannot be expected, thereby providing inexpensive, high-performance sintered ferrite magnets containing reduced amounts of Co and La.

The value of n represents a molar ratio of (Fe+Co) to (Ca+R+A), 2n=(Fe+Co)/(Ca+R+A). The molar ratio n is preferably 4≦n≦7. When n is less than 4, the magnet contains a large percentage of non-magnetic portions with excessively flat calcined particles, resulting in drastically reduced H_(cJ). When n is more than 7, the unreacted α-Fe₂O₃ remains in the calcined body, resulting in the leakage of a slurry from a mold cavity during wet molding. The range of n is preferably 4.5≦n≦6, more preferably 4.8≦z≦5.2.

Si is an indispensable element, and contained in an amount of more than 1% by mass and 1.8% or less by mass (calculated as SiO₂) based on the entire magnet. Si is preferably added in the form of SiO₂ to the calcined ferrite, and the amount of SiO₂ added is preferably more than 1% by mass and 1.8% or less by mass based on 100% by mass of the calcined body. SiO₂ added to the calcined body essentially remains in the sintered magnet, too, without change. It should be noted, however, that some of SiO₂ may flow out in the pulverization step and the molding step, resulting in a smaller amount in the sintered magnet than in the calcined body. Si essentially forms the grain boundary phase, without being contained in the ferrite phase having a hexagonal M-type magnetoplumbite structure. The preferred range of Si is 1.1-1.6% by mass (calculated as SiO₂).

A molar ratio range x/z of the element R to Co is preferably 0.73≦x/z≦15.62, more preferably 1≦x/z≦3, most preferably 1.2≦x/z≦2. By selecting the composition meeting these values, the magnetic properties can be improved.

In the present invention, large effects of improving magnetic properties are obtained when the amount of the element R>the amount of Co>the amount of the element A, namely, x>z>y. Also, when the amount of Ca>the amount of the element A, namely, (1-x-y)>y, large effects of improving magnetic properties are obtained.

The sintered ferrite magnet of the present invention, and the calcined body used for producing it have a ferrite phase having a hexagonal M-type magnetoplumbite structure. The term “having a hexagonal M-type magnetoplumbite structure” used herein means that the X-ray diffraction pattern of the hexagonal M-type magnetoplumbite structure is mainly observed when the X-ray diffraction of the sintered ferrite magnet or the calcined body is measured under general conditions. There may be undesired phases (orthoferrite phase, spinel phase, etc.) and impurity phases in extremely small amounts (about 5% or less by mass) observed by X-ray diffraction, etc. The quantitative measurement of undesired phases by X-ray diffraction can be conducted by a Rietveld analysis method.

The sintered ferrite magnet of the present invention indispensably comprises the ferrite phase having a hexagonal M-type magnetoplumbite structure, and the grain boundary phase indispensably containing Si. Because the grain boundary phase cannot easily be observed by an X-ray diffraction pattern, it is preferably confirmed by a transmission electron microscope, etc.

In the present invention, both the calcined ferrite and the sintered ferrite magnet have the same ferrite phase, which has a hexagonal M-type magnetoplumbite structure. The calcined ferrite preferably contains the ferrite phase as a main component for the orientation of crystals in the molding and structure control in the sintering, the existence of the grain boundary phase being not critical. On the other hand, the sintered ferrite magnet contains the ferrite phase as a main component, and needs the grain boundary phase indispensably containing Si for structure control and densification in the sintering process.

With the above preferred ranges, the sintered ferrite magnet of the present invention has magnetic properties such as coercivity H_(cJ) of 450 kA/m or more, a residual magnetic flux density B_(r) of 0.4 T or more, and a squareness ratio H_(k)/H_(cJ) of 80% or more. With more preferred ranges, it has magnetic properties such as coercivity H_(cJ) of 460 kA/m or more, a residual magnetic flux density B_(r) of 0.44 T or more, and a squareness ratio H_(k)/H_(cJ) of 80% or more.

[2] Production Method of Sintered Ferrite Magnet

The sintered ferrite magnet is produced by a step of preparing calcined ferrite, a step of pulverizing the calcined ferrite to powder, a step of molding the powder to green body, and a step of sintering the green body to a sintered body. Before the pulverization step, more than 1% by mass and 1.8% or less by mass of SiO₂ can be added to 100% by mass of the calcined body to provide the sintered magnet with remarkably improved coercivity H_(cJ).

The calcined ferrite comprises a ferrite phase having a hexagonal M-type magnetoplumbite structure comprising Ca, the element R which is at least one of rare earth elements and indispensably includes La, the element A which is Ba and/or Sr, Fe, and Co as indispensable elements, the composition of metal elements of Ca, R, A, Fe and Co being represented by the general formula of Ca_(1-x-y)R_(x)A_(y)Fe_(2n-z)CO_(z), wherein the atomic ratios of Ca (1-x-y), the element R (x), the element A (y) and Co (z), and the molar ratio of n meet the following relations: 0.3≦(1-x-y)≦0.65, 0.2≦x≦0.65, 0≦y≦0.2, 0.03≦z≦0.65, and 4≦n≦7.

The composition of the calcined ferrite including 0 (oxygen) is represented by the general formula of Ca_(1-x-y)R_(x)A_(y)Fe_(2n-z)CO_(z)O_(α), wherein (1-x-y), x, y, z, α represents the atomic ratios of Ca, the element R, the element A, Co and O, and n represents a molar ratio, meeting 0.3≦(1-x-y)≦0.65, 0.2≦x≦0.65, 0≦y≦0.2, 0.03≦z≦0.65, and 4≦n≦7, and α=19 at a stoichiometric composition ratio in which the element R and Fe are trivalent, Co is bivalent, x =z, and n=6.

In the composition of the above calcined ferrite including O (oxygen), the mole ratio of oxygen may differ depending on the valences of Fe and Co, the value of n, and the kind of the element R. Also, in the sintered ferrite magnet, a ratio of oxygen to the metal element may change depending on the vacancy of oxygen when sintered in a reducing atmosphere, the change of the valences of Fe and Co in the ferrite phase, etc. Accordingly, the actual molar ratio α of oxygen may be deviated from 19. Therefore, the composition of metal elements, which are most easily identified, are used in the present invention.

(1) Step of Preparing Calcined Ferrite

The calcined ferrite is produced by mixing oxide powders and powders of compounds such as Ca compounds, compounds of the elements R, Ba and/or Sr compounds, if necessary, iron compounds, and Co compounds, which are converted to oxides by calcining, to the above composition; and calcining (ferritizing) the resultant mixture. The composition ranges of Ca, the element R, the element A, Fe and Co are restricted for the same reasons as in the case of the sintered ferrite magnet.

Usable as the starting material powders are powders of metals in the form of oxides, carbonates, hydroxides, nitrates, chlorides, etc., regardless of their valences. Solutions of starting material powders may also be used. Used as the Ca compounds are carbonates, oxides, chlorides, etc. of Ca. Used as the compounds of the elements R are oxides such as La₂O₃, hydroxides such as La(OH)₃, carbonates such as La₂(CO₃)₃.8H₂O, etc. Particularly, the oxides, hydroxides, carbonates, etc. of mixed rare earth elements (La, Nd, Pr, Ce, etc.) are preferable because they are inexpensive, enabling cost reduction. Used as the compounds of the elements A are carbonates, oxides, chlorides, etc. of Ba and/or Sr. Used as the iron compounds are iron oxides, iron hydroxides, iron chlorides, mill scales, etc. Used as the Co compounds are oxides such as CoO, CO₃O₄, etc., hydroxides such as CoOOH, Co(OH)₂, CO₃O₄.m₁H₂O (m₁ is a positive number), etc., carbonates such as CoCO₃, and basic carbonates such as m₂CoCO₃.m₃Co(OH)₂.m₄H₂O, etc. (m₂, m₃ and m₄ are positive numbers).

Other starting material powders than CaCO₃, Fe₂O₃ and La₂O₃ may be added at the time of mixing starting materials or after the calcination. For example, the sintered ferrite magnet can be produced, (1) by mixing CaCO₃, Fe₂O₃, La₂O₃ and CO₃O₄, calcining the resultant mixture, and pulverizing, molding and sintering the calcined body, or (2) by mixing CaCO₃, Fe₂O₃ and La₂O₃, calcining the resultant mixture, adding CO₃O₄ to the calcined body, and pulverizing, molding and sintering the calcined body.

To accelerate a reaction during calcining, about 1% or less by mass of a B-containing compound such as B₂O₃, H₃BO₃, etc. may be added, if necessary. Particularly, the addition of H₃BO₃ is effective for improving H_(cJ) and B_(r). The amount of H₃BO₃ added is more preferably 0.3% or less by mass, most preferably about 0.2% by mass. Less than 0.1% by mass of H₃BO₃ provides only a small effect of improving B_(r), and more than 0.3% by mass of H₃BO₃ reduces B_(r). Because H₃BO₃ has effects of controlling the shapes and sizes of crystal grains during sintering, it may be added after calcining (before fine pulverization or before sintering), or both before and after calcining.

The mixing of the starting material powders may be conducted in a wet or dry state. When stirred with media such as steel balls, etc., the starting material powders can be mixed more uniformly. In the case of wet mixing, the solvent is preferably water. To increase the dispersibility of the starting material powders, known dispersants such as ammonium polycarboxylate, calcium gluconate, etc. may be used. A slurry of the starting materials is dewatered to obtain mixed starting material powders.

The mixed starting material powders are heated in an electric furnace, a gas furnace, etc. to cause a solid-phase reaction, thereby forming a ferrite compound having a hexagonal M-type magnetoplumbite structure. This process is called “calcining,” and the resultant compound is called “calcined body.”

The calcining step is conducted preferably in an atmosphere having an oxygen concentration of 5% or more. The oxygen concentration of less than 5% results in abnormal grain growth, the formation of undesired phases, etc. More preferably, the oxygen concentration is 20% or more.

In the calcining step, the solid-phase reaction forming the ferrite phase proceeds as the temperature is elevated, and is completed at about 1100° C. When the calcining temperature is lower than 1100° C., the unreacted hematite (iron oxide) remains, resulting in low magnetic properties. On the other hand, when the calcining temperature is higher than 1450° C., crystal grains grow excessively, likely needing a long period of time for the pulverization step. Accordingly, the calcining temperature is preferably 1100-1450° C., more preferably 1200-1350° C. The calcining time is preferably 0.5-5 hours.

When H₃BO₃ is added before calcining, the ferritization reaction is accelerated, enabling calcining at 1100-1300° C.

(2) Addition of SiO₂

The production method of the present invention is characterized by adding more than 1% by mass and 1.8% or less by mass of SiO₂ to 100% by mass of the calcined body before the pulverization step. This specifically improves H_(cJ). The addition of 1% or less by mass of SiO₂ fails to obtain an effect of improving H_(cJ), and the addition of more than 1.8% by mass of SiO₂ undesirably reduces H_(cJ), B_(r) and H_(k)/H_(cJ). The more preferred amount of SiO₂ added is 1.1-1.6% by mass.

Though SiO₂ is most preferably added to the calcined body, part of SiO₂ may be added before calcining (when mixing the starting material powders). With SiO₂ added before calcining, the size control of crystal grains can be conducted in calcining.

(3) Addition of CaCO₃

Depending on the amount of SiO₂ added, 1-2% by mass (calculated as CaO) of CaCO₃ is preferably added to 100% by mass of the calcined body before the pulverization step. The addition of CaCO₃ prevents the decrease of B_(r) and H_(k)/H_(cJ) as much as possible, resulting in sintered ferrite magnets having high H_(cJ) with high B_(r) and H_(k)/H_(cJ) maintained, which have not been achieved conventionally. When the amount of CaCO₃ (calculated as CaO) is less than 1% by mass or more than 2% by mass, B_(r) and H_(k)/H_(cJ) are undesirably low. The amount of CaCO₃ added is more preferably 1.2-2% by mass.

The preferred amount of SiO₂ added may slightly change depending on the amount of CaCO₃ (calculated as CaO). As shown in Examples below, it slightly changes also depending on the amount (z) of Co. For example, from the aspect of improving H_(cJ), increase in the amount of CaCO₃ added tends to shift the preferred amount of SiO₂ to a higher side, regardless of the amount (z) of Co. Also, decrease in the amount of Co tends to shift the preferred amount of SiO₂ to a higher side. However, too much SiO₂ reduces B_(r) and H_(k)/H_(cJ). To achieve high H_(cJ) while maintaining high B_(r) and H_(k)/H_(cJ), it is preferable to add SiO₂ in an amount of 1.1-1.5% by mass and CaCO₃ in an amount of 1.2-2% by mass (calculated as CaO) when the amount (z) of Co is z≦0.3, and to add SiO₂ in an amount of 1.4-1.6% by mass and CaCO₃ in an amount of 1.5-2% by mass (calculated as CaO) when the amount (z) of Co is z<0.3. Considering both cases, as described above, the amount of SiO₂ added is preferably 1.1-1.6% by mass. In this case, the amount (calculated as CaO) of CaCO₃ added is preferably 1.2-2% by mass.

In the present invention, when both SiO₂ and CaCO₃ are added, the amounts of SiO₂ and CaCO₃ may be properly determined within the above ranges. As described above, it is preferable to add SiO₂ in a range of 1.1-1.6% by mass and CaCO₃ in a range of 1.2-2% by mass (calculated as CaO), providing sintered ferrite magnets with high H_(cJ), with high B_(r) and H_(k)/H_(cJ) maintained.

When both SiO₂ and CaCO₃ are added, the magnetic properties can be improved by adjusting a ratio of the amount (calculated as CaO) of CaCO₃/the amount of SiO₂ to 0.8-2. In this case, the preferred range of [CaCO₃ (as CaO)/SiO₂] slightly changes depending on the amount (z) of Co as shown in Examples below. When Z≧0.3, the ratio is preferably 1-1.7, more preferably 1.1-1.4. When z<0.3, the ratio is preferably 0.8-1.4, more preferably 0.9-1.1. Considering both cases of z≧0.3 and z<0.3, the ratio is preferably 0.8-1.7, more preferably 0.9-1.4. With [CaCO₃ (as CaO)/SiO₂] set in the above range, the sintered ferrite magnets has high H_(cJ), with high B_(r) and H_(k)/H_(cJ) maintained.

(4) Pulverization Step

The calcined body is pulverized by a vibration mill, a ball mill, an attritor, etc. to powder. The pulverized powder preferably has an average particle size of about 0.4-0.8 μm (measured by an air permeation method). The pulverization step may be either dry pulverization or wet pulverization, though their combination is preferable.

The wet pulverization uses water and/or non-aqueous solvents (organic solvents such as acetone, ethanol, xylene, etc.). The wet pulverization produces a slurry containing the calcined body in water (solvent). The slurry preferably contains known dispersant and/or surfactant in an amount of 0.2-2% by mass on a solid basis. After the wet pulverization, the slurry is preferably concentrated and kneaded.

In the pulverization step, Cr₂O₃, Al₂O₃, etc. may be added together with SiO₂ and CaCO₃ described above to improve the magnetic properties. Each of them is preferably 5% or less by mass.

Because the pulverized powder contains ultra-fine powder of less than 0.1 μm, which cause poor dewatering and molding defects, the pulverized powder is preferably heat-treated to remove the ultra-fine powder. The heat-treated powder is preferably pulverized again. Thus, using a pulverization step comprising a first fine pulverization step, a step of heat-treating powder obtained by the first fine pulverization step, and a second fine pulverization step of pulverizing the heat-treated powder again, which is called “heat-treating, repulverizing step,” H_(cJ) can be further improved in addition to the effect of improving H_(cJ) by the addition of SiO₂ and CaCO₃, thereby providing sintered ferrite magnets with extremely high H_(cJ), with high B_(r) and H_(k)/H_(cJ) maintained, which have not been obtained so far.

Ultra-fine powder of less than 0.1 μm is likely to be inevitably formed in a usual pulverization step, and the existence of ultra-fine powder lowers H_(cJ), results in poor dewatering in the molding step, provides green bodies with defects, and deteriorates pressing cycles because too much time is needed for dewatering. When the powder containing ultra-fine powder, which is obtained by the first fine pulverization step, is heat-treated, a reaction occurs between powder having relatively large particle sizes and the ultra-fine powder, resulting in decrease in the amount of the ultra-fine powder. In the second fine pulverization step, the particle sizes are controlled with necking removed, to produce powder with predetermined particle sizes. Thus, powder having an excellent particle size distribution with a small percentage of ultra-fine powder can be obtained, thereby improving H_(cJ), and thus solving the above problems in the molding step.

Utilizing the effect of improving H_(cJ) by the heat-treating, repulverizing step, the same H_(cJ) as when using powder having an average particle size of about 0.4-0.8 μm, which is produced by a usual pulverization step, is obtained, even if powder obtained by the second fine pulverization step has relatively large particle sizes (for example, an average particle size of about 0.8-1.0 μm). Accordingly, time saving can be achieved by the second fine pulverization step, with improved dewatering and pressing cycle.

As described above, though the heat-treating, repulverizing step provides various advantages, cost increase due to increased production steps cannot be avoided. However, magnetic properties are much more improved by the heat-treating, repulverizing step in the production of the sintered ferrite magnet of the present invention than in the production of conventional sintered ferrite magnets, canceling the above cost increase. Accordingly, the heat-treating, repulverizing step is practically significant in the present invention.

The first fine pulverization may be the same as the above-described usual pulverization, using a vibration mill, a jet mill, a ball mill, an attritor, etc. The pulverized powder preferably has an average particle size of about 0.4-0.8 μm (measured by an air permeation method). The pulverization step may be either dry pulverization or wet pulverization, though their combination is preferable.

After the first fine pulverization step, a heat treatment is conducted preferably at 600-1200° C., more preferably 800-1100° C. Though not particularly restricted, the heat treatment time is preferably 1 second to 100 hours, more preferably about 1-10 hours.

The second fine pulverization after the heat treatment step uses a vibration mill, a jet mill, a ball mill, an attritor, etc., like the first fine pulverization. Because the desired particle sizes are already substantially obtained by the first fine pulverization step, the control of particle sizes and the removal of necking are mainly conducted in the second fine pulverization step. Accordingly, milder pulverization conditions due to the shortened pulverization time, etc. are preferably used in the second fine pulverization step than in the first fine pulverization step. Pulverization under the same conditions as in the first fine pulverization step undesirably forms ultra-fine powder again.

The average particle size of the powder after the second fine pulverization is preferably about 0.4-0.8 μm (measured by an air permeation method) as in a usual pulverization step when seeking higher H_(cJ) than that of sintered ferrite magnets obtained by the usual pulverization step, and preferable 0.8-1.2 μm, more preferably about 0.8-1.0 μm (measured by an air permeation method) when seeking the saving of time in the pulverization step, improving dewatering and a pressing cycle, etc.

(5) Molding Step

A slurry after the pulverization is press-molded with or without a magnetic field while removing water (solvent). Press-molding in a magnetic field aligns the crystal orientation of powder particles, thereby drastically improving the magnetic properties. To improve the orientation further, a dispersant and a lubricant may be added in an amount of 0.01-1% by mass. Also, the slurry may be concentrated before molding, if necessary. The concentration is preferably conducted by centrifugal separation, filter pressing, etc.

(6) Sintering Step

The press-molded green body is degreased if necessary, and then sintered. The sintering is conducted in an electric furnace, a gas furnace, etc.

The sintering is conducted preferably in an atmosphere having an oxygen concentration of 10% or more. When the oxygen concentration is less than 10%, abnormal grain growth, the formation of undesired phases, etc. occur, resulting in deteriorated magnetic properties. The oxygen concentration is more preferably 20% or more, most preferably 100%.

The sintering temperature is preferably 1150-1250° C. The sintering time is preferably 0.5-2 hours. The sintered magnet has an average crystal particle size of about 0.5-2 μm.

After the sintering step, the sintered ferrite magnet is obtained through known production steps such as a machining step, a washing step, an inspection step, etc.

The present invention will be explained in more detail referring to Examples without intention of restricting it thereto.

EXAMPLE 1

CaCO₃ powder, La(OH)₃ powder, Fe₂O₃ powder and CO₃O₄ powder were mixed to have the formula of Ca_(1-x-y)La_(x)A_(y)Fe_(2n-z)CO_(z)O_(19-δ), wherein x=0.5, y=0, z=0.3, n=5.2, and 6≧0, and 0.1% by mass of H₃BO₃ powder was added to the total amount (100% by mass) of the mixed powders to produce a starting material powder. This starting material powder was mixed by a wet ball mill for 4 hours, and granulated by drying. It was calcined at 1300° C. for 3 hours in the air, and the calcined body was coarsely pulverized by a hammer mill to obtain coarse powder.

SiO₂ powder and CaCO₃ powder (calculated as CaO) were added in the amounts shown in Table 1 to the above coarse powder, and fine pulverization was conducted by a wet ball mill using water as a solvent until its average particle size (measured by an air permeation method) became 0.55 μm. Each of the resultant fine powder slurries was molded in a magnetic field of about 1.3 T under pressure of about 50 MPa with a compression direction parallel to the direction of the magnetic field, while removing the solvent. Each of the resultant green bodies was sintered at 1200° C. for 1 hour in the air to obtain a sintered magnet.

Each sintered magnet was measured with respect to a residual magnetic flux density B_(r), coercivity H_(cJ), and a squareness ratio H_(k)/H_(cJ), wherein H_(k) was the value of H at J of 0.95B_(r) on a curve of J (intensity of magnetization) to H (intensity of a magnetic field) in the second quadrant. The measurement results are shown in FIGS. 1-3. In FIGS. 1-3, the axes of abscissas represent the amount (% by mass) of SiO₂ added, and the axes of ordinates represent a residual magnetic flux density B_(r) (T) (FIG. 1), coercivity H_(cJ) (FIG. 2) and a squareness ratio H_(k)/H_(cJ) (FIG. 3). In the figures, data obtained with the same amount of CaO were connected by straight lines. A dotted line at H_(k)/H_(cJ)=80% in FIG. 3 is a line as a measure for practical use. Even if a sintered magnet had high B_(r) and H_(cJ), it would not be able to be made thin when its H_(k)/H_(cJ) is less than 80%, substantially failing to be useful for various applications such as electronic parts for automobiles, parts for electric appliances, etc. The same is true of Examples below.

TABLE 1 Sample CaO SiO₂ No. (% by mass) (% by mass) CaO/SiO₂ 101 0.7 0.0 — 102 0.7 0.3 2.33 103 0.7 0.6 1.17 104 1.0 0.0 — 105 1.0 0.3 3.33 106 1.0 0.6 1.67 107 1.0 0.9 1.11  108* 1.0 1.2 0.83  109* 1.3 1.1 1.18 110 1.5 0.3 5.00 111 1.5 0.6 2.50 112 1.5 0.9 1.67  113* 1.5 1.2 1.25  114* 1.5 1.5 1.00 115 1.5 1.9 0.80 116 2.0 0.6 3.33 117 2.0 0.9 2.22  118* 2.0 1.2 1.67  119* 2.0 1.5 1.33  120* 2.0 1.8 1.11 Note: *Within the present invention.

As is clear from FIGS. 1-3, high magnetic properties are obtained when more than 1% by mass of SiO₂ and 1% or more by mass of CaO are added, and higher magnetic properties are obtained when 1.1-1.5% by mass of SiO₂ and when 1.3-2% by mass of CaO are added. Particularly, Sample 113 to which 1.2% by mass of SiO₂ and 1.5% by mass of CaO were added, and Sample 119 to which 1.5% by mass of SiO₂ and 2.0% by mass of CaO were added had specifically improved H_(cJ) with maximally suppressed decrease in B_(r) and H_(k)/H_(cJ).

FIG. 4 compares Samples 113 and 119, and samples containing 1% or less by mass of SiO₂, which are shown in FIG. 2. It is clear that as compared with samples to which 1% or less by mass of SiO₂ was added, which have conventionally been considered optimum, Sample 113 to which 1.2% by mass of SiO₂ and 1.5% by mass of CaO were added, and Sample 119 to which 1.5% by mass of SiO₂ and 2.0% by mass of CaO were added had H_(cJ) improved by about 20% or more.

As described above, because Ca—La—Co ferrite in which the atomic ratio of Co was 0.3 had an anisotropic magnetic field H_(A) of 2.1 MA/m, the H_(cJ) (400 kA/m or less) of the samples containing 1% or less by mass of SiO₂ was about 19% or less of the above anisotropic magnetic field H_(A), but the H_(cJ) of Samples 113 and 119 was about 23% of the above anisotropic magnetic field H_(A), closer to the inherent potential of the material. Further, taking into consideration Samples 113 and 119 having CaO/SiO₂ of 1.25 and 1.33, and Sample 109 having better magnetic properties, it has been found that excellent magnetic properties can be obtained at a ratio of CaO/SiO₂ in a range of about 1.1-1.4.

FIG. 5 compares Sample 103 (CaO=0.7% by mass, and SiO₂=0.6% by mass), Sample 107 (CaO=1.0% by mass, and SiO₂=0.9% by mass), Sample 109 (CaO=1.3% by mass, and SiO₂=1.1% by mass), Sample 113 (CaO=1.5% by mass, and SiO₂=1.2% by mass), and Sample 119 (CaO=2.0% by mass, and SiO₂=1.5% by mass), which exhibit the highest H_(cJ) at the CaO content of 0.7% by mass, 1.0% by mass, 1.3% by mass, 1.5% by mass, and 2.0% by mass, respectively, in FIG. 2. In FIG. 5, the axis of abscissas represents the amount (% by mass) of CaO added, and the axis of ordinates represents coercivity H_(cJ). As is clear from FIG. 5, as the amount of CaO added increases, the coercivity H_(cJ) is improved. Particularly when the amount of CaO added was 1.5% by mass and 2% by mass, the highest coercivity H_(cJ) was obtained. Also, when the amount of CaO added was in a range of 1.2-2% by mass, the coercivity H_(cJ) was about 20% or more of the anisotropic magnetic field H_(A) of Ca—La—Co ferrite in which the atomic ratio of Co was 0.3, and when the amount of CaO added was in a range of 1.5-2% by mass, the coercivity H_(cJ) was about 23% of the above anisotropic magnetic field H_(A).

With respect to the sintered magnets of Samples 113 and 114, the amounts of SiO₂ and CaO added and those measured by inductively coupled plasma (ICP) atomic emission spectroscopy are shown in Table 2. The amounts of SiO₂ and CaO added are expressed by “% by mass” determined based on the entire composition.

TABLE 2 Amount of SiO₂ Amount of CaO Sample (% by mass) (% by mass) No. Measured Added Measured Added 113 1.19 1.17 3.98 4.37 114 1.66 1.46 3.96 4.36

As shown in Table 2, SiO₂ added to the calcined body remained in the sintered magnet. The amount of SiO₂ measured was larger than that added, presumably because the amounts of other elements than SiO₂ became smaller than when added.

EXAMPLE 2

Sintered magnets were produced in the same manner as in Example 1, except that SiO₂ powder and CaCO₃ powder (calculated as CaO) were added in the amounts shown in Table 3 to a composition having the formula of Ca_(1-x-y)La_(x)A_(y)Fe_(2n-z)CO_(z)O_(19-δ), wherein x=0.5, y=0, z=0.2, n=4.8, and δ≧0.

The sintered magnets were measured with respect to a residual magnetic flux density B_(r), coercivity H_(cJ), and a squareness ratio H_(k)/H_(cJ). The measurement results are shown in FIGS. 6-8. As in Example 1, in FIGS. 6-8, the axes of abscissas represent the amount (% by mass) of SiO₂ added, and the axes of ordinates represent a residual magnetic flux density B_(r) (T) (FIG. 6), coercivity H_(cJ) (FIG. 7), and a squareness ratio H_(k)/H_(cJ) (FIG. 8), data at the same CaO content being connected by a straight line.

TABLE 3 Sample CaO SiO₂ No. (% by mass) (% by mass) CaO/SiO₂ 201 0.6 0.2 3.00 202 0.6 0.4 1.50 203 0.6 0.6 1.00 204 0.6 0.8 0.75 205 0.6 1.0 0.60 206 1.0 0.2 5.00 207 1.0 0.4 2.50 208 1.0 0.6 1.67 209 1.0 0.8 1.25 210 1.0 1.0 1.00 211 1.5 1.0 1.50  212* 1.5 1.2 1.25  213* 1.5 1.4 1.07  214* 1.5 1.5 1.00  215* 1.5 1.6 0.94  216* 1.5 1.8 0.83 217 1.5 2.0 0.75 218 2.0 1.0 2.00  219* 2.0 1.5 1.33 220 2.0 2.0 1.00 Note: *Within the present invention.

As is clear from FIGS. 6-8, even in the compositions in which the amount of Co was changed, as in Example 1, high magnetic properties were obtained when more than 1% by mass of SiO₂ and 1% or more by mass of CaO were added, and better magnetic properties were obtained when 1.4-1.6% by mass of SiO₂ and 1.5-2% by mass of CaO were added. Particularly, Sample 215 to which 1.6% by mass of SiO₂ and 1.5% by mass of CaO were added had specifically improved H_(cJ), while suppressing decrease in B_(r) and H_(k)/H_(cJ) as much as possible.

FIG. 9 compares Sample 215 to which 1.6% by mass of SiO₂ and 1.5% by mass of CaO were added, and samples to which 1% or less by mass of SiO₂ was added, which are shown in FIG. 7. As is clear from FIG. 9, Sample 215 to which 1.6% by mass of SiO₂ and 1.5% by mass of CaO were added had H_(cJ) improved by about 27%, as compared with samples containing 1% or less by mass of SiO₂, which have conventionally been considered optimum.

SPD measurement revealed that Ca—La—Co ferrite in which the atomic ratio of Co was 0.2 had an anisotropic magnetic field H_(A) of 1.9 MA/m (about 23.9 kOe). Accordingly, samples to which 1% or less by mass of SiO₂ was added had H_(cJ) (300 kA/m or less), which was about 16% or less of the above anisotropic magnetic field H_(A), Sample 215 to which 1.6% by mass of SiO₂ and 1.5% by mass of CaO were added had H_(cJ), which was about 20% of the above anisotropic magnetic field H_(A), close to the inherent potential of the material. Further, CaO/SiO₂ was 0.94 in Sample 215. It was found that excellent magnetic properties were obtained at a CaO/SiO₂ ratio in a range of about 0.9-1.1.

EXAMPLE 3

Sintered magnets were produced in the same manner as in Example 1, except that SiO₂ powder and CaCO₃ powder (calculated as CaO) were added in the amounts shown in Table 4 to a composition having the formula of Ca_(1-x-y)La_(x)A_(y)Fe_(2n-z)CO_(z)O_(19-δ), wherein x=0.5, y=0, z=0.25, n=5.0, and δ≧0.

The sintered magnets were measured with respect to a residual magnetic flux density B_(r), coercivity H_(cJ), and a squareness ratio H_(k)/H_(cJ). The measurement results are shown in FIGS. 10-12. As in Example 1, in FIGS. 10-12, the axes of abscissas represent the amount (% by mass) of SiO₂ added, and the axes of ordinates represent a residual magnetic flux density B_(r) (T) (FIG. 10), coercivity H_(cJ) (FIG. 11), and a squareness ratio H_(k)/H_(cJ) (FIG. 12). To investigate influence by the Co content (z) in FIGS. 10-12, Samples 111-115 in Example 1 and Samples 211-217 in Example 2 were plotted, and data at the same Co content (z) were connected by a straight line.

TABLE 4 Sample CaO SiO₂ No. (% by mass) (% by mass) CaO/SiO₂ 301 1.5 0.9 2.00  302* 1.5 1.2 1.33  303* 1.5 1.5 1.00 304 1.5 1.9 0.78 Note: *Within the present invention.

As is clear from FIGS. 10-12, even at the Co content (z) of 0.25, as in Examples 1 and 2, high magnetic properties were obtained when more than 1% by mass of SiO₂ and 1% or more by mass of CaO were added. Particularly when SiO₂ was 1.5% by mass, H_(cJ) was specifically improved while suppressing decrease in B_(r) and H_(k)/H_(cJ) as much as possible.

It is also clear from FIGS. 10-12 that the preferred amount of SiO₂ differs at z of 0.3, 0.25 and 0.2, and that as the Co content (z) decreases, the preferred amount of SiO₂ tends to shift toward a higher side. Namely, the amount of SiO₂ providing high H_(cJ) while maintaining high B_(r) and H_(k)/H_(cJ) was in a range of 1.1-1.2% by mass when the Co content (z) was 0.3, and in a range of 1.4-1.6% by mass when the Co content (z) was 0.25 and 0.2, respectively.

FIG. 13 shows these data, the axis of abscissas representing CaO/SiO₂, and the axis of ordinates representing coercivity H_(cJ), and data at the same Co content (z) being connected by a straight line. As is clear from FIG. 13, when both SiO₂ and CaO are added, magnetic properties can be improved by setting CaO/SiO₂ in a range of 0.8-2.0. In this case, it is clear that the preferred range of CaO/SiO₂ slightly differs between when z=0.3 and when z<0.3 (z=0.25, z=0.2). Namely, the range of CaO/SiO₂ for achieving high H_(cJ) while maintaining high B_(r) and H_(k)/H_(cJ) is preferably 1-1.7, more preferably 1.2-1.4, when z=0.3, and preferably 0.8-1.4, more preferably 0.9-1.1, when z<0.3 (z=0.25, z=0.2). Considering both cases of z≧0.3 and z<0.3, it is preferably 0.8-1.7, more preferably 0.9-1.4.

It is clear from FIG. 11 that a lower Co content (z) tends to provide a lower maximum value of H_(cJ). However, as considered in Examples 1 and 2, H_(cJ) was improved by 20% or more in the same Co content (z), as compared with samples to which 1% or less by mass of SiO₂ (conventionally considered optimum) was added, the improvement of H_(cJ) being 4% or more of the anisotropic magnetic field H_(A). It was thus confirmed that the sintered magnets had H_(cJ) close to potential inherent in the materials. Accordingly, when magnets having the H_(cJ) same as that of conventional sintered Ca—La—Co ferrite magnets are provided, the amounts of rare and expensive Co and La can be reduced. As in Example 2, even for compositions having a Co content z of 0.2, which are not expected to have high H_(cJ) so that it has been difficult to put them into practical use, H_(cJ) on a practical level can be obtained, thereby providing high-performance sintered ferrite magnets with reduced amounts of Co and La at a low cost.

EXAMPLE 4

Sintered magnets were produced in the same manner as in Example 1, except that CaCO₃ powder, La(OH)₃ powder, SrCO₃ powder, BaCO₃ powder, Fe₂O₃ powder and CO₃O₄ powder were mixed to provide the formula of Ca_(1-x-y1-y2)La_(x)Sr_(y1)Ba_(y2)Fe_(2n-z)CO_(z)O_(19-δ), wherein x=0.5, y1+y2=0.05, z=0.3, n=5.3, and δ≧0, and that the amounts of SiO₂ powder and CaCO₃ powder (calculated as CaO) relative to the total amount of these mixed powders were changed as shown in Table 5. The atomic ratios of Sr (y1) and Ba (y2) are also shown in Table 5. Samples 111-115 are sintered magnets evaluated in Example 1.

The sintered magnets were measured with respect to a residual magnetic flux density B_(r), coercivity H_(cJ) and a squareness ratio H_(k)/H_(cJ), wherein H_(k) is the value of H, at which J is 0.95B_(r), on a curve of J (intensity of magnetization) to H (intensity of a magnetic field) in the second quadrant. The measurement results are shown in FIGS. 14-16. In FIGS. 14-16, the axes of abscissas represent the amount (% by mass) of SiO₂ added, and the axes of ordinates represent a residual magnetic flux density B_(r) (T) (FIG. 14), coercivity H_(cJ) (FIG. 15) and a squareness ratio H_(k)/H_(cJ) (FIG. 16), respectively. In the figures, data connected by a straight line are denoted by “Unsubstituted” representing Samples 111-115, “Ba” representing Samples 401-404 to which only Ba was added, “Sr” representing Samples 405-408 to which only Sr was added, and “Sr+Ba” representing Samples 409-412 to which both Sr and Ba were added.

TABLE 5 Sample Atomic Ratio Atomic Ratio CaO (% SiO₂ (% CaO/ No. of Sr (y1) of Ba (y2) by mass) by mass) SiO₂ 401 0 0.05 1.5 0.6 2.50 402 0 0.05 1.5 0.9 1.67  403* 0 0.05 1.5 1.2 1.25  404* 0 0.05 1.5 1.5 1.00 405 0.05 0 1.5 0.9 1.67  406* 0.05 0 1.5 1.2 1.25  407* 0.05 0 1.5 1.5 1.00 408 0.05 0 1.5 1.9 0.80 409 0.025 0.025 1.5 0.9 1.67  410* 0.025 0.025 1.5 1.2 1.25  411* 0.025 0.025 1.5 1.5 1.00 412 0.025 0.025 1.5 1.9 0.80 111 0 0 1.5 0.6 2.50 112 0 0 1.5 0.9 1.67  113* 0 0 1.5 1.2 1.25  114* 0 0 1.5 1.5 1.00 115 0 0 1.5 1.9 0.80 Note: *Within the present invention.

As is clear from FIGS. 14-16, the compositions to which Sr and/or Ba were added also had high magnetic properties when more than 1% by mass of SiO₂ and 1% or more by mass of CaO were added, as in Examples 1-3 not containing Sr and Ba.

FIG. 17 is a graph showing these data plotted with the axis of abscissas being CaO/SiO₂, and the axis of ordinates being coercivity H_(cJ). As is clear from FIG. 17, the compositions to which Sr and/or Ba were added also had high magnetic properties at a CaO/SiO₂ ratio of 0.8-2.0 when both SiO₂ and CaO were added, as in Examples 1-3 not containing Sr and Ba.

EXAMPLE 5

Sintered magnets were produced in the same manner as in Example 1, except that SrCO₃ powder, La(OH)₃ powder, Fe₂O₃ powder and CO₃O₄ powder were mixed to have the formula of Sr_(1-x)La_(x)Fe_(2n-z)CO_(z)O_(19-δ), wherein x=0.2, z=0.2, n=5.8, and δ≧0, and that the amounts of SiO₂ powder and CaCO₃ powder (calculated as CaO) were changed as shown in Table 6. These sintered magnets are so-called Sr—La—Co ferrite magnets having basic compositions outside the present invention.

The sintered magnets were measured with respect to a residual magnetic flux density B_(r), coercivity H_(cJ), and a squareness ratio H_(k)/H_(cJ). The measurement results are shown in FIGS. 18-20.

TABLE 6 Sample CaO SiO₂ No. (% by mass) (% by mass) CaO/SiO₂ 501 0.7 0.0 — 502 0.7 0.3 2.33 503 0.7 0.6 1.17 504 1.0 0.0 — 505 1.0 0.3 3.33 506 1.0 0.6 1.67 507 1.0 0.9 1.11 508 1.0 1.2 0.83 509 1.5 0.3 5.00 510 1.5 0.6 2.50 511 1.5 0.9 1.67 512 1.5 1.2 1.25 513 1.5 1.5 1.00

It is clear from FIGS. 18-20 that when more than 1% by mass of SiO₂ was added, Sr—La—Co ferrite magnets do not have good magnetic properties, with H_(cJ) not extremely improved, and the squareness ratio lowered to less than 80%.

EXAMPLE 6

CaCO₃ powder, La(OH)₃ powder, Fe₂O₃ powder and CO₃O₄ powder were mixed to have the formula of Ca_(1-x-y)La_(x)A_(y)Fe_(2n-z)CO_(z)O_(19-δ), wherein x=0.5, y=0, z=0.3, n=5.2, and δ≧0, and 0.1% by mass of H₃BO₃ powder was added to 100% by mass of the mixed powders to obtain a starting material powder. This starting material powder was mixed for 4 hours by a wet ball mill, and granulated by drying. It was calcined at 1300° C. for 3 hours in the air, and the calcined body was pulverized to coarse powder by a hammer mill.

1.2% by mass of SiO₂ powder and 1.5% by mass (calculated as CaO) of CaCO₃ powder were added to the coarse powder, and the first fine pulverization was conducted to an average particle size of 0.5 μm (measured by an air permeation method) by a wet ball mill with water as a solvent. The resultant powder was dried, and heat-treated at 1000° C. for 5 hours in the air. The heat-treated powder had an average particle size of 1.4 μm (measured by an air permeation method). The heat-treated powder was subject to the second fine pulverization to an average particle size of 0.8 μm by a wet ball mill. While removing the solvent, a fine powder slurry obtained by the above heat-treating, repulverizing step was molded in a magnetic field of about 1.3 T under pressure of about 50 MPa, with a compression direction parallel to the direction of a magnetic field. The resultant green body was sintered at 1200° C. for 1 hour in the air to obtain a sintered magnet.

In addition, a sintered magnet was produced in the same manner as above, except for conducting only the first fine pulverization to an average particle size of 0.8 μm (measured by an air permeation method) without the heat treatment and the second fine pulverization.

The sintered magnets were measured with respect to a residual magnetic flux density B_(r), coercivity H_(cJ), and a squareness ratio H_(k)/H_(cJ), wherein H_(k) was the value of H at J of 0.95B_(r) on a curve of J (intensity of magnetization) to H (intensity of a magnetic field) in the second quadrant. The measurement results are shown in Table 7. Table 7 shows Sample 601 pulverized in the heat-treating, repulverizing step, and Sample 602 pulverized only in the first fine pulverization.

TABLE 7 Sample Br H_(cJ) Hk/H_(cJ) No. (T) (kA/m) (%) 601 0.438 494.0 80.1 602 0.436 387.9 80.4

As is clear from Table 7, the use of the heat-treating, repulverizing step drastically improved H_(cJ) with B_(r) and H_(k)/H_(cJ) unchanged, than the use of only one usual pulverization.

Sample 113 in Example 1 shown in Table 1 was obtained in the same manner as in this Example except for using only one usual pulverization to an average particle size of 0.55 μm, having magnetic properties of H_(cJ)=476 kA/m, Br=0.443 T, and H_(k)/H_(cJ)=81.6% as shown in FIGS. 1-3. Comparison of this sample with Sample 601 in this Example revealed that Sample in this Example had slightly improved H_(cJ) with B_(r) and H_(k)/H_(cJ) substantially unchanged than the sample (average particle size 0.55 μm) in Example 1, despite the fact that the pulverized powder had a large average particle size (0.8 μm). Namely, even though powder after the second fine pulverization step has relatively large particle sizes, the use of the heat-treating, repulverizing step provides as small H_(cJ) as when using powder having relatively small particle sizes, which is obtained by a usual pulverization step. Accordingly, the second fine pulverization step can be conducted in a shortened period of time, thereby improving dewatering and pressing cycles.

Effect of the Invention

Because the production method of the present invention can extremely improve H_(cJ) while maintaining high B_(r) and squareness ratio H_(k)/H_(cJ), even if the amounts of rare and expensive Co and La indispensable for Ca—La—Co ferrite are reduced than before, economically advantageous Ca—La—Co ferrite magnets can be obtained. Ca—La—Co ferrite magnets obtained by the production method of the present invention do not suffer demagnetization by a demagnetizing field generated by thinning the magnet. Accordingly, the sintered ferrite magnet of the present invention can be used for electronic parts for automobiles and parts for electric appliances, such as small, light-weight, high-performance motors, power generators, speakers, etc. 

What is claimed is:
 1. A method for producing a sintered ferrite magnet comprising (a) a ferrite phase having a hexagonal M-type magnetoplumbite structure, and (b) a grain boundary phase indispensably containing Si, said method comprising a step of preparing calcined ferrite containing a ferrite phase having a hexagonal M-type magnetoplumbite structure, which comprises Ca, an element R which is at least one of rare earth elements and indispensably includes La, an element A which is Ba and/or Sr, Fe, and Co as indispensable elements, the composition of metal elements of Ca, R, A, Fe and Co being represented by the general formula of Ca_(1-x-y)R_(x)A_(y)Fe_(2n-z)Co_(z), wherein the atomic ratios of Ca (1-x-y), the element R (x), the element A (y) and Co (z), and the molar ratio of n meet the following relations: 0.42≦(1-x-y)≦0.5, 0.45≦x≦0.55, 0≦y≦0.8, 0.2≦z≦0.3, and 4.8≦n≦5.2; a step of pulverizing said calcined body to powder; a step of molding said powder to a green body; and a step of sintering said green body to obtain a sintered body; more than 1% by mass and 1.8% or less by mass of SiO₂ and 1-2% by mass (calculated as CaO) of CaCO₃ being added to 100% by mass of said calcined body, before the pulverization step.
 2. The method for producing a sintered ferrite magnet according to claim 1, wherein the amount of SiO₂ added is 1.1-1.6% by mass.
 3. The method for producing a sintered ferrite magnet according to claim 1, wherein the amount of CaO added is 1.2-2% by mass.
 4. The method for producing a sintered ferrite magnet according to claim 1, wherein said pulverization step comprises a first fine pulverization step, a step of heat-treating powder obtained by said first fine pulverization step, and a second fine pulverization step of pulverizing the heat-treated powder again.
 5. A sintered ferrite magnet comprising (a) a ferrite phase having a hexagonal M-type magnetoplumbite structure comprising Ca, an element R which is at least one of rare earth elements and indispensably includes La, an element A which is Ba and/or Sr, Fe, and Co as indispensable elements, the composition of metal elements of Ca, R, A, Fe and Co being represented by the general formula of Ca_(1-x-y)R_(x)A_(y)Fe_(2n-z)CO_(z), wherein the atomic ratios of Ca (1-x-y), the element R (x), the element A (y) and Co (z), and the molar ratio of n meet the following relations: 0.42≦(1-x-y)≦0.5, 0.45≦x≦0.55, 0≦y≦0.8, 0.2≦z≦0.3, and 4.8≦n≦5.2, and (b) a grain boundary phase indispensably containing Si, the amount of said Si being more than 1% by mass and 1.8% or less by mass (calculated as SiO₂) based on the entire sintered ferrite magnet.
 6. The sintered ferrite magnet according to claim 5, wherein the amount of Si is 1-1.6% by mass (calculated as SiO₂) based on the entire sintered ferrite magnet. 